PWM relay actuator circuit

Abstract

An actuation circuit for an electromechanical relay employs a microprocessor and a switching transistor to actuate a relay over a wide range of applied voltages. The switching transistor is connected in series with the actuator coil. A voltage sensing circuit is connected to one input of the microprocessor, which produces a pulse-width modulated actuator output to gate the switching transistor. The microprocessor is suitably programmed so as to produce a pulse width modulated signal actuator signal whose duty cycle is a function of applied voltage. The RMS current through the relay coil is sufficient to ensure good actuation, but does not overstress the coil. The relay is selected to have its rated actuation voltage at or below a lower end of the expected range of applied voltages. A relay actuation boost arrangement can include a transistor or an SUS (silicon unidirectional switch) or similar negative resistance device.

We claim: 
     
       1. Actuator control circuit for energizing an actuator coil over a wide range of operating voltages, comprising: source means for applying the actuating voltage across a pair of conductors;   a switching transistor having a control electrode and a pair of current electrodes, the current electrodes being connected in series with the actuator coil;   a microprocessor having a control output coupled to the control electrode of said switching transistor;   voltage sensing means connected across said pair of conductors, and having an output applying a voltage signal to a voltage input of said microprocessor, said voltage signal being proportional to the actuating voltage across said conductors within said range of operating voltages;   said microprocessor being programmed so as to produce, when actuation is called for, an actuator signal appearing at said control output which is pulse width modulated to have a duty cycle that is a linear function of said actuating voltage within said range of operating voltage.   
     
     
       2. Actuator control circuit according to claim 1, wherein said actuator is selected to have its actuation voltage at a lower end of said range. 
     
     
       3. Actuator control circuit according to claim 1, wherein said microprocessor is programmed to sense when relay actuation is called for, to sense if the voltage signal is above a lower limit, and if so to compute for said output signal a pulse width modulation duty cycle as a linear function of said voltage signal. 
     
     
       4. Actuator control circuit according to claim 1, wherein said microprocessor is programmed to generate at said control output a DC level for a predetermined time at commencement of said actuator signal. 
     
     
       5. Actuator control circuit according to claim 4, wherein said microprocessor is programmed to generate at said control output a DC level for a predetermined time at the termination of said actuator signal. 
     
     
       6. Actuator control circuit according to claim 1, further comprising actuation boost means for storing flyback voltage from pulsed operation of said switching transistor and applying an increased actuation voltage across the actuator coil at commencement of said actuator signal. 
     
     
       7. Actuator control circuit for energizing an actuator coil over a wide range of operating voltages, comprising: source means for applying the actuating voltage across a pair of conductors;   a switching transistor having a control electrode and a pair of current electrodes, the current electrodes being connected in series with the actuator coil;   a microprocessor having a control output coupled to the control electrode of said switching transistor;   voltage sensing means connected across said pair of conductors, and having an output applying a voltage signal to a voltage input of said microprocessor, said voltage signal being proportional to the actuating voltage across said conductors within said range of operating voltages;   said microprocessor being programmed so as to produce, when actuation is called for, an actuator signal appearing at said control output which is pulse width modulated to have a duty cycle that is a linear function of said actuating voltage within said range of operating voltage; and   actuation boost means for applying an increased actuation voltage across the actuator coil of said relay at commencement of said actuator signal;   wherein said actuation boost means includes a storage capacitor and a second switching device connected in series, a diode coupled on one side to a junction of said actuator coil and the first-mentioned switching transistor and on the other side to a junction of the storage capacitor and said second switching device to charge said storage capacitor with flyback voltage from said actuator coil, said second switching device being coupled to the actuator coil so that an increased actuation voltage stored on said storage capacitor passes to said actuator coil at commencement of the actuator signal.   
     
     
       8. Actuator control circuit according to claim 7, wherein said second switching device is a transistor having a control electrode coupled to an output of said microprocessor and a current electrode coupled to the relay actuator coil. 
     
     
       9. Actuator control circuit according to claim 7, wherein said second switching device includes a negative resistance device connected between said storage capacitor and the relay actuator coil.